The present invention relates generally to electric machine drives and more particularly to an electric machine drive having sensorless control.
The elimination of rotor position, velocity, and flux sensors from electric machine drives has significant cost, reliability, and competitive advantages. Chief among these is the reduction in the cost associated with these expensive sensors and their associated wiring and connectors, which together are one of the leading causes of problems in electric machines and one of the most expensive to fix. These sensors and their associated wiring and connectors are also a major reliability concern due to their inherent fragile nature and sensitivity to harsh environments. The elimination of these sensors also provides a competitive advantage beyond those related to cost and reliability by allowing higher performance and efficiency control techniques to be used in applications where lower performance control techniques would have been used previously due to a lack of these sensors.
The control techniques used to eliminate rotor position, velocity or flux sensors are often described as xe2x80x9csensorless control.xe2x80x9d At high rotor speeds and excitation frequencies, these techniques have shown a lot of promise and have achieved some commercial success. Their inability to run at lower speeds and excitation frequencies excludes these techniques from being used in applications that require operation in these regions, including industrial drives, consumer appliances, and automotive applications (for example, electric traction drives for electric or fuel cell powered vehicles, hybrid electric vehicles, and electric power assisted steering).
To overcome the inability of operating at low rotor speeds and excitation frequencies, a second class of xe2x80x9csensorless controlsxe2x80x9d techniques has been developed that eliminates the dependence on the fundamental excitation of the electric machine. These techniques use a separate, high frequency sensing excitation applied to the machine in addition to fundamental excitation. There are many forms that the sensing excitation can take, but a fundamental requirement for those techniques to work is that the electric machine must have some form of saliency. For rotor position or velocity estimation, the saliency must have been related to and move when the rotor of the machine moves. For flux estimation, the saliency typically has some saturation-related dependence and hence contains information related to the position of the flux in the machine. Some examples of these machine types include synchronous reluctance machines, buried permanent magnet machines, switch reluctance machines, and salient pole synchronous machines.
Other machine types, for example induction machines, do not inherently have significant rotor position dependent saliencies present. In fact, it is often one of the design goals for machines such as these to minimize the magnitudes of the saliencies present. Due to this lack of an inherent rotor position dependent saliency, creating a saliency in these types of machines without significantly affecting the fundamental operation and torque production of the machine is necessary before rotor position or velocity xe2x80x9csensorless controlxe2x80x9d can be implemented.
Many methods are used to modify or design an induction machine rotor with a rotor position dependent saliency. For example, one method to create a rotor position dependent saliency is to vary the width of the rotor bar slot openings in a repeating manner. The variable width slot openings create a saliency by modulating the reluctance seen by the leakage flux as a function of the rotor position. The variation in width is done in such a way so that a complete cycle of the variation has a period equal to the pole pitch of the machine. Such a period results in a coupling between the saliency and the fundamental of the machine stator windings.
A second method used to create a rotor position dependent saliency is the modify the depth of fill that the rotor bars occupy in the rotor bar slot openings in a repeating pattern with a period again equal to the pole pitch of the machine. This creates a saliency due to the fact that the aluminum or copper occupying the rotor bar slot openings will have eddy currents induced in them due to the high frequency sensing signal injected into the machine for the xe2x80x9csensorless control.xe2x80x9d Because the amount of this fill in the rotor bar slot openings varies around the circumference of the rotor, the amount of eddy currents induced varies and produces the appearance of a high frequency saliency. This is akin to the use of a double rotor cage to increase the amount of starting line torque in line-started induction machines.
A third method of creating a rotor position dependent saliency is to vary the cross-sectional area of the rotor bars, thereby causing the rotor resistance to have a spatial dependence. One of the distinguishing features of this method from the other methods currently used for creating a rotor position dependent saliency is that it creates the saliency using the rotor resistance, whereas the other methods create a saliency by causing the leakage inductance to vary. As with the other methods for creating a saliency, the variation in rotor bar cross-sectional area is done so that it repeats every pole pitch of the machine.
A fourth method for creating a rotor position dependent is to create a saliency by selecting the number of rotor bars and stator slots so that their combination results in the desired saliency. For example, the combination of 28 rotor bars and 24 stator slots results in a rotor position dependent saliency that repeats 4 times around the circumference of the machine and would have a period equal to the pole pitch of the machine in a 4 pole machine.
The methods listed previously for creating a rotor position dependent saliency share several disadvantages in the way and type of saliency they create. First, all of the methods tie the desired saliency to the torque producing mechanism of the machine. Since the rotor bars play a fundamental role in the torque produced by the machine, the sensing and torque producing functions become inherently tied together. Some of the disadvantages of this relationship include increased ripple torque, increased clogging torque, increased sensitivity to saturation, and a limited potential for modification due to the number of rotor bars.
Another significant limitation caused by creating the desired saliency through the rotor bar design is that skewing of the rotor bars, as is often done in induction machines, significantly reduces any saliency created by these methods. Further, these methods tend to create relatively small magnitude saliencies, making it more difficult to use them for xe2x80x9csensorless control.xe2x80x9d
To overcome some of the disadvantages and limitations of the methods described above, a new method for creating rotor dependent saliencies in electric drive machines is presented.
The new method introduces a plurality of sensing slots around the outer periphery of the rotor. These sensing slots either directly or coupled with the plurality of stator slots create a rotor dependent desired saliency. The sensing slots effectively decouple the sensing and torque producing functions in an electric drive motor. This significantly reduces the potential for ripple torque, clogging torque, or saturation effects.
By varying the width, depth, and spacing within the sensing slots, the saliency of the various electric drive machines can be varied quite easily to produce a desired effect.
The advantages to creating a rotor position dependent saliency using separate sensing slots are numerous and significant. As described above, the sensing function or torque producing function is separated in the electric drive machine. Also, since the sensing slots are not tied to the rotor bars in any way, skewing of the rotor bars does not reduce the saliency magnitude significantly. Further, the effects of saturation on the saliency are minimized since the fundamental flux near the sensing slots will typically be parallel to the sensing slots.
Another significant advantage to creating a rotor dependent saliency using separate sensing slots is that the invention works equally well in pre-existing or new machine designs. Also, as the sensing slots can be introduced in a post-production step, wherein there is no need to keep track of the orientation of laminations during the assembly process, manufacturing techniques are greatly simplified, thereby decreasing the costs associated with manufacturing the rotor.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.